The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The use of physical sensors in transient fluid flow applications such as the exhaust system of an engine is challenging due to harsh environmental conditions such as vibration and thermal cycling. One known temperature sensor includes a mineral insulated sensor inside a thermowell that is then welded to a support bracket, which retains a tubular element. This design, unfortunately, takes a long amount of time to reach stability, and high vibration environments can result in damage to physical sensors.
Physical sensors also present some uncertainty of the actual resistive element temperature in many applications, and as a result, large safety margins are often applied in the design of heater power. Accordingly, heaters that are used with physical sensors generally provide lower watt density, which allows a lower risk of damaging the heater at the expense of greater heater size and cost (same heater power spread over more resistive element surface area).
Moreover, known technology uses an on/off control or PID control from an external sensor in a thermal control loop. External sensors have inherent delays from thermal resistances between their wires and sensor outputs. Any external sensor increases the potential for component failure modes and sets limitations of the any mechanical mount to the overall system.
One application for heaters in fluid flow systems is vehicle exhausts, which are coupled to an internal combustion engine to assist in the reduction of an undesirable release of various gases and other pollutant emissions into the atmosphere. These exhaust systems typically include various after-treatment devices, such as diesel particulate filters (DPF), a catalytic converter, selective catalytic reduction (SCR), a diesel oxidation catalyst (DOC), a lean NOx trap (LNT), an ammonia slip catalyst, or reformers, among others. The DPF, the catalytic converter, and the SCR capture carbon monoxide (CO), nitrogen oxides (NOx), particulate matters (PMs), and unburned hydrocarbons (HCs) contained in the exhaust gas. The heaters may be activated periodically or at a predetermined time to increase the exhaust temperature and activate the catalysts and/or to burn the particulate matters or unburned hydrocarbons that have been captured in the exhaust system.
The heaters are generally installed in exhaust pipes or components such as containers of the exhaust system. The heaters may include a plurality of heating elements within the exhaust pipe and are typically controlled to the same target temperature to provide the same heat output. However, a temperature gradient typically occurs because of different operating conditions, such as different heat radiation from adjacent heating elements, and exhaust gas of different temperature that flows past the heating elements. For example, the downstream heating elements generally have a higher temperature than the upstream elements because the downstream heating elements are exposed to fluid having a higher temperature that has been heated by the upstream heating elements. Moreover, the middle heating elements receive more heat radiation from adjacent upstream and downstream heating elements.
The life of the heater depends on the life of the heating element that is under the harshest heating conditions and that would fail first. It is difficult to predict the life of the heater without knowing which heating element would fail first. To improve reliability of all the heating elements, the heater is typically designed to be operated with a safety factor to avoid failure of any of the heating elements. Therefore, the heating elements that are under the less harsh heating conditions are typically operated to generate a heat output that is much below their maximum available heat output.